1. Field of the Invention
The present invention relates to a thread forming screw to be set in a hard component such as concrete and having a stem, and a thread profile provided on at least a portion of a longitudinal extent of the stem and formed as a trapezoidal thread having a crest region, a root region, and two flanks connecting opposite ends of the crest region with the root region and tapering outwardly toward the root region, with each flank being formed of outer and inner flank sections with a flank angle formed by the outer flank sections being smaller than a flank angle formed by the inner flank sections.
2. Description of the Prior Art
Screws of the type described above serve for securing objects on hard constructional components such as, e.g., concrete. To this end, a bore is formed in the constructional component in which the screw is set. Upon being screwed into the bore, the thread profile provided on the screw stem cuts or rolls a thread in the wall of the bore. Because of the hardness and brittleness of the constructional component, the thread profile of the screw should have a specific shape.
A screw of the type discussed above is disclosed in German Utility Model DE-U1-93 02 186. The trapezoidal thread of the screw has a root region, a crest region that defines the outer diameter of the screw thread, and flanks that connect the crest region with the root region and are formed of outer and inner flank sections forming different flank angles toward the root region, with the outer flank sections forming an angle of 30xc2x0 and the inner flank section forming an angle of 60xc2x0. By forming the inner flank sections with a greater flank angle than the outer flank sections, advantageously, the thread is reinforced in the root region due to the increase of the material thickness in this region. In particular for hard constructional components, providing a thread with a reinforced root region increases substantially the load-carrying capability of the screw disclosed in DE-01-93 02 186.
However, because of the high friction between the bore wall and the crest region of the thread, the screw does not have adequate screw-in characteristics.
Moreover, the manufacturing of this screw is rather expensive as several steps are required in order to provide the screw with the desired thread profile.
Accordingly, an object of the present invention is to provide a thread forming screw having adequate screw-in characteristics.
Another object of the present invention is to provide a thread forming screw that can be economically produced.
These and other objects of the present invention, which will become apparent hereinafter, are achieved by forming the crest region of a first region section extending parallel to a central axis of the screw and a second, inclination region section inclined toward the root region in the direction opposite the setting direction at an angle from about 10xc2x0 to about 45xc2x0.
The provision of the inclination region section in the crest region permits to improve the flow characteristics of a material, e.g., steel which is used for forming the screw. The improved flow characteristic permits, in turn to obtain a precise geometry of the thread and, as a result, optimal screw-in characteristics. On the other hand, providing an inclined flank section permits to obtain, during the setting of the inventive screw into the constructional component, a so-called xe2x80x9csnow plow effectxe2x80x9d which consists in that the material, which is comminuted by the outer crest region, is carried away by the inclination section, and is compressed by an excess material thickness section resulting from the increased flank angle formed by the inner flank sections. The first region section of the crest region functions as a chisel, removing the wall material.
To insure optimal cutting off, comminuting, and carrying away the wall material, advantageously the extent of the crest region in the setting direction corresponds to about 1-4 times of the extent of the inclination region section in this direction. Dependent on the screw dimensions and operational characteristics, the extent of the region can correspond (coinside) substantially with the extent of the inclination region section.
Advantageously, the root region has a two-sided recess forming a root angle of about 150xc2x0-170xc2x0. This angle proved to be optimal to insure that sufficient material is available for formation of the thread. Otherwise, an excess accumulation of the comminuted material in the recess would take place, preventing the increase of friction between the screw and the wall of the bore upon the screw being screwed-in.
In an advantageous embodiment of the inventive screw, an equal-sided recess is formed. This permits to precisely define, during the manufacturing of the inventive screw, the yield limit.
To obtain optimal screw-in characteristics, the thread profile is advantageously formed as a double thread, with the two-section flanks being provided on the first, in the setting direction, thread, with the outer flank sections forming a flank angle of from about 30xc2x0 to about 50xc2x0 and the inner flank sections forming a flank angle from about 60xc2x0 to about 90xc2x0.
For manufacturing reasons, the second thread is formed with flanks having a flank angle of from about 60xc2x0 to about 90xc2x0.
In order to obtain, during setting of the inventive screw, an optimal ratio between the material carrying-away step and the compression step of the comminuted material, the thread is so formed that the maximum outer diameter of the first thread preferably corresponds to from 1.1 to 1.3 times of the maximum outer diameter of the second thread.
Further, preferably, the break point between first and second flank section is so selected that its diameter corresponds approximately to 1.2 times of the core diameter of the screw.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.